The role of viruses or viral vaccines as possible teratogens is under investigation. An attenuated Influenza A Virus Vaccine under development for human use as well as the wild strain of Reo I Virus have been fround to cause fetal death or severe Central Nervous System anomalies when inoculated intracerebrally into 100-day-old rhesus monkey fetuses. None of the control animals used in the studies developed malformations. Additional studies are in progress in which the pregnant animal rather than the fetus is infected much earlier in gestation.